1. Field of the Invention
The present invention relates to an optical element, a method for producing the same, and an optical pickup comprising the optical element.
2. Description of the Related Art
Recently, it is demanded to make an optical disc storage with medium high density. Therefore, in optical disc apparatuses, research and development have been underway for shortening the wavelength of a light source and for enlarging the numerical aperture (NA) of a reproduction optical disc system. Further, it is desired to increase the transfer rate of data in such the reproduction optical disc systems.
When shortening the wavelength of the light source and enlarging the NA of the reproduction optical disc system, in addition to the fact that the size of an optical spot becomes small, the focal depth becomes shallower, so it is desired to reduce the focus servo error. Further, a data recording width (track width) of the optical disc storage medium becomes narrower, so it is desired to reduce the tracking servo error.
For increasing the data transfer rate, it is desired to make the bandwidth of an actuator used for the focus servo and tracking servo higher. As a result, two improvements are desired for servo characteristics: reduction of the remaining amount of error and raising the bandwidth. An actuator can be improved in its servo characteristics by reducing the weight of the actuator.
FIG. 1A is a sectional view illustrating an optical element as a related art and FIG. 1B is a plane view of the optical element in FIG. 1A.
This optical element 10 has a substrate 14A formed by an optical material and having a thickness t1. The substrate 14A has a convex part 11 configuring a convex lens and a flat part 12 positioned around the convex part 11.
The optical element 10 can be formed by heat treating a circular mask layer with a Radius R formed on the front surface of a planar optical material to form it into a lens shape by surface tension, then etching the planar optical material so that the lens shape is transferred to the optical material.
At the outer circumference of the convex part 11 is formed a groove 19 called a xe2x80x9ctrenchxe2x80x9d formed when transferring the shape by the etching. The groove 19 clearly separates the convex part 11 and the flat part 12.
In the optical element 10 in FIGS. 1A and 1B, parts other than the convex part 11 are etched away to give a thin planar shape.
When using this optical element 10 in combination with other optical elements, care must be taken that an optical path in other combined optical elements is maintained.
FIGS. 2A and 2B are sectional views of the optical element 10 in FIGS. 1A and 1B and a lens holder thereof. FIG. 2A shows a case when clamping the circumferential edge of the flat part 12 of the optical element 10 by a lens holder 10A. FIG. 2B shows a case when clamping an upper surface of the flat part 12 of the optical element 10 by a lens holder 10B.
As shown in FIGS. 2A and 2B, it is necessary to enlarge the flat part 12 in order to mount the optical element 10 in FIGS. 1A and 1B on the lens holders 10A and 10B.
Further, considering deviation of position at the time of bonding and a squeezing out of excess adhesive, it is necessary to enlarge the flat part 12 not only in the case when using the optical element 10 in combination with other optical elements. For example, when the convex part 11 has an effective diameter (diameter) of a convex lens of about 200 xcexcm and has a radius of curvature of the convex lens of about 150 xcexcm, the thickness of the flat part 12 becomes about 50 xcexcm or less. If assuming the sum of tolerance of the positioning accuracy in the bonding process and the amount of the adhesive squeezed out to be about 500 xcexcm, the convex lens ends up being positioned on a thin plate of about 50 xcexcm thickness and about 500 xcexcm length.
Therefore, improvement of mechanical strength is desired for the optical element as shown in FIGS. 1A and 1B. Further, because the flat part 12 is thin, resonance due to vibration in a thickness direction easily occurs. Therefore, it is desired to make the structure difficult to resonate.
An object of the present invention is to provide an optical element having a substrate formed by an optical material, with the improved mechanical strength and free from a critical resonance.
Another object of the present invention is to provide an optical pickup having the optical element.
Still another object of the present invention is to provide a method for producing the optical element with efficiency and at a high accuracy.
According to a first aspect of the present invention, there is provided an optical element having a substrate formed by an optical material, wherein the substrate has a convex part functioning as a convex lens, a flat part positioned around the convex part, and an outer circumference part positioned around the flat part, the thickness of the outer circumference part being greater than that of the flat part.
Preferably, the thickness of the outer circumference part is thicker than that of the convex part.
Preferably, the substrate has a first groove formed at a boundary between the convex part and the flat part, for defining a region of the convex part.
Preferably, the substrate has a second groove formed at a boundary between said flat part and said outer circumference part, for defining a region of said flat part.
Preferably, the optical material comprises fused silica.
Preferably, the surface of the outer circumference part is flat or approximately flat.
Preferably, a plurality of steps are formed at the outer circumference part, and the thickness of the substrate at an outer side step is thicker than that of the substrate at an inner side step.
According to the first aspect of the present invention, there is provided an optical element having a substrate formed by an optical material, wherein the substrate has a convex part functioning as a convex lens, a flat part positioned around the convex part, an outer circumference part positioned around the flat part, a first groove formed at a boundary between the convex part and the flat part, for defining a region of the convex part, and a second groove formed at a boundary between the flat part and the outer circumference part, for defining a region of the flat part, wherein a thickness of the outer circumference part is greater than that of the flat part, both the flat part and the outer circumference part have flat shapes in the thickness direction, and the convex part, the flat part, and the outer circumference part are integrated in a unit by the substrate.
According to the first aspect of the present invention, there is provided an optical element having a substrate formed by an optical material, wherein the substrate has a convex part functioning as a convex lens, a flat part positioned around the convex part, an outer circumference part positioned around the flat part, and a groove formed at a boundary between the convex part and the flat part, for defining a region of the convex part, wherein a thickness of the outer circumference part is greater than that of the convex part, both the flat part and the outer circumference part have flat shapes in the thickness direction, and the convex part, the flat part, and the outer circumference part are integrated in a unit by the substrate.
According to the first aspect of the present invention, there is provided an optical element having a substrate formed by an optical material, wherein the substrate has a convex part functioning as a convex lens, a flat part positioned around the convex part, a first outer circumference part positioned around the flat part, a second outer circumference part positioned around the first outer circumference part, a third groove formed at a boundary between the convex part and the flat part, for defining a region of the convex part, and a fourth groove formed at a boundary between the flat part and the first outer circumference part, for defining a region of the flat part, wherein the thickness of the first and second outer circumference parts are greater than that of the flat part and the thickness of the second outer circumference part is greater than that of the first outer circumference part, all the flat part and the first and second outer circumference parts have flat shapes in the thickness direction, and the convex part, the flat part, and the first and second outer circumference parts are integrated in a unit by the substrate.
According to a second aspect of the present invention, there is provided a method for producing an optical element comprising the steps of forming a first mask layer and a second mask layer surrounding the first mask layer on a substrate formed by an optical material, heat treating the first mask layer to form it into a convex lens shape, and etching the substrate to transfer the convex lens shape of the first mask layer to the substrate.
Preferably, in the step of forming the first and second mask layers, the first and second mask layers are formed by patterning a mask layer formed by a photosensitive material on the substrate.
Preferably, in the step of heat treating the first mask layer to form it into a convex lens shape, the heat treatment temperature is higher than a glass transition temperature of the first mask layer.
Preferably, in the step of heat treating the first mask layer to form it into a convex lens shape, the heat treatment temperature is lower than a carbonization temperature of the first mask layer.
Preferably, in the step of heat treating the first mask layer to form it into a convex lens shape, the heat treatment temperature is higher than room temperature or ordinary temperature.
Preferably, in the step of forming the first and second mask layers, the second mask layer having an opening part is formed, then the first mask layer is formed at the opening part. In this case, the second mask layer is formed by an etching-resistant material.
Preferably, the second mask layer comprises a third mask layer formed by an etching-resistant material and a fourth mask layer superposed on the third mask layer so as to cover the third mask layer on the substrate. In this case, the fourth mask layer is formed by the same material as the first mask layer.
Preferably, the substrate is formed by fused silica, the first and fourth mask layers are formed by an optically transparent material having a good characteristic for forming a thick film, and the second and third mask layers are formed by platinum.
According to a third aspect of the present invention, there is provided an optical pickup having an optical element functioning as an object lens when mounted on a recording and/or reproducing apparatus of an optical storage medium and a photodetector for receiving a reflected light beam for use in recording and/or reproduction to and from the optical storage medium, the optical element comprising a substrate formed by an optical material, the substrate comprising a convex part functioning as a convex lens, a flat part positioned around the convex part, and an outer circumference part positioned around the flat part, a thickness of the outer circumference part being thicker than that of the flat part to thereby improve mechanical strength and increase a resonance frequency.
Preferably, the thickness of the outer circumference part is thicker than that of the convex part.
Preferably, a surface of the outer circumference part is flat or approximately flat. Holding of the optical element is eased by this.
Preferably, a plurality of steps are formed at the outer circumference part and the thickness of the substrate at an outer side step is thicker than that of the substrate at an inner side step. An eclipse can be prevented thereby.
The substrate comprised of an optical material comprises a convex part functioning as a convex lens, a flat part positioned around the convex part, and an outer circumference part positioned around the flat part. Since the thickness of this outer circumference part is greater than that of the flat part, in comparison with the same thickness, the mechanical strength of the substrate can be improved and it is possible to improve the mechanical strength and reliability of the optical element.
In the optical element according to the present invention, since the outer circumference part is formed thick, it is possible to make the flat part thinner than the outer circumference part and thus increase the mechanical strength. Further, the flexural strength is also increased because of the reduction of the thin flat part, the resonance frequency in the thickness direction is heightened, and thus it is possible to make the optical element a structure resistant to resonance.
Further, in the optical element according to the present invention, it is possible to enlarge the size of the optical element due to the reduction of the thin flat part. Due to this, the allowable amount of excess adhesive squeezed out can be enlarged, so it is both possible to facilitate the mounting on a lens holder and to increase the diameter of an attachment part formed in the lens holder. The requirement on precision also can be relaxed, so it is possible to relax the requirement on precision of the lens holder mounting this optical element.
According to the method for producing the optical element according to the present invention, when forming the convex part, the thick outer circumference part can be formed without increasing or almost without increasing the steps of the process. Further, since the positioning accuracy of the patterning of the mask layer can be maintained, the convex part and the outer circumference part can be fabricated with a high positioning accuracy. As a result, the flat part positioned around the convex part can be made smaller and, for example, the flat part can be made narrower to the resolution of the material of the mask layer.
Further, according to the method for producing an optical element according to the present invention, by using an etching-resistant material for the material of the mask layer of the outer circumference part, the outer circumference part can be made thicker than the convex part and it is possible to further improve the mechanical strength and to further raise the resonance frequency.
Further, by using a stacked structure mask layer as the mask layer of the outer circumference part, it is possible to form an optical element having a multistep structure at the thick outer circumference part and possible to make the structure one in which the outer circumference part does not easily block the optical path, so the thick outer circumference part can be formed up to near the convex part and it is possible to further improve the mechanical strength and to further raise the resonance frequency.
Further, according to the optical pickup of the present invention, it is possible to provide an optical pickup having an optical element according to the above present invention.